In conventional semiconductor devices, operation speed is improved mainly by miniaturizing the structure thereof. For example, in a high electron mobility transistor, the diffusion of impurities can be effectively prevented and the operating speed can be improved by using two-dimensionally distributed electrons. However, in such semiconductor devices, signal transmission is conducted by the migration of a charge (electrons), and the migration speed is restricted by the saturation speed of the material. Therefore, there is a limitation to any attempt to shorten the channel transit time, making high-speed operation difficult. Under these circumstances, in order to realize high-speed operation, the structure of the device must be further miniaturized. Therefore, it has been reported that, for example, a micro-machining technique at a scale of 0.1 μm or less is required for forming the gate of a device so that high-speed operation at hundreds of GHz can be achieved.
To overcome such a drawback, a semiconductor device wherein the charge transit is not directly involved in signal transmission has been proposed. For example, document 1 (Phys. Rev. Lett. V. 71(1993), p. 2465) and document 2 (IEEE Trans. Elect. Dev. V. 43(1996), p. 380), both written by M. Dyakonov and M. Shur, propose a device using the plasma oscillation of a highly concentrated electron fluid in an FET channel. FIG. 7 is a cross-sectional view of the FET device of document 1. Document 3 (Japanese Unexamined Patent Publication No. 1996-139306) proposes an electromagnetic wave amplifying device that uses the phenomenon of binary instability that occurs in a binary system comprising positive holes and electrons or heavy electrons and light electrons. FIG. 8 is a perspective view of the electromagnetic wave amplifying device disclosed in document 3.
In the prior art techniques, operation at a high frequency band is theoretically feasible; however, they are not very practical because of drawbacks such as a limited operating range, difficulties in the input and output of signals, a complicated structure, etc. For example, in the structure of the devices disclosed in documents 1 and 2, it is reported that electromagnetic wave oscillation at a high frequency, frequency multiplication, and detection of electromagnetic waves are possible; however, these were not fully verified by experiments except for the detection of an initial response. One of the factors preventing the practical application of these techniques is that, for example, in the FET device disclosed in document 1 and shown in FIG. 7, it is difficult to couple the input/output of the signals with the plasma oscillation in the device.
Japanese Unexamined Patent Publication No. 2000-294768 discloses a semiconductor device as described below in which leak current passing through an insulating film can be reduced. In the device, an n-type electron supply layer made of AlGaN and an n-type electron transit layer made of GaN are sequentially formed on a sapphire substrate through a buffer layer and a base layer each formed of undoped AlGaN. A gate electrode 17 is formed on the electron transit layer with an insulating film made of AlN in between. The insulating film is grown at a maximum temperature of 900° C., and composed of a plurality of columnar crystal grains. The maximum depth of the gaps between the columnar crystal grains on the surface of the insulating film on the gate electrode side is 80% or less of the thickness of the insulating film, and their average depth is 35% or less of the film thickness. The average diameter of the columnar crystal grains is equal to or less than 40 nm.
Japanese Unexamined Patent Publication No. 2000-252458 discloses a semiconductor device as described below that is capable of suppressing leakage current passing through an insulating film. In the device, on a sapphire substrate, an n-type electron feeding layer made of AlGa and an n-type electron transit layer made of GaN are formed sequentially with a buffer layer and a base layer each formed of undoped AlGaN in between. A gate electrode is formed on the electron transit layer with an insulating film in between. The insulating film has a structure in which a first AlN insulating film and a second SiO2 insulating film are laminated sequentially on the electron transit layer. Leakage current, which is generated if only the first insulating film is formed, can be prevented by providing the second insulating film.
Japanese Unexamined Patent Publication No. 1996-274346 discloses a semiconductor in which high-performance amplifier and mixer circuits are obtained by forming a dual-gate FET that has improved characteristics due to an enhanced gate breakdown withstand voltage strength. To achieve such a structure, the intrinsic partial conductance during voltage control in the dual-gate FET is made larger when the first gate electrode is used than when the second gate electrode is used. At the same time, the drain breakdown withstand voltage strength of the second gate electrode is made larger than that of the first gate electrode. Deterioration of mutual conductance is thereby eliminated to improve the withstand voltage of the device.
The present invention aims to solve the above problems and to provide a plasma oscillation switching device that reliably achieves operation at a high frequency band.